Determination of Inorganic Anions in Environmental Waters Using a Hydroxide-Selective Column

Importance of the Topic

Accurate measurement of inorganic anions in environmental waters is critical for compliance with drinking water and pollution control regulations. Ion chromatography is a widely accepted technique under regulations such as the U.S. Safe Drinking Water Act and Clean Water Act for monitoring contaminants like fluoride, chloride, nitrite, nitrate, phosphate and sulfate.

Objectives and Study Overview

This study evaluated the performance of a hydroxide‐selective IonPac AS18 column coupled with reagent-free electrolytic generation of potassium hydroxide for U.S. EPA Method 300.0. Key targets included assessing separation quality, method detection limits (MDLs), calibration linearity, precision, and analyte recoveries in various environmental water matrices.

Methodology and Used Instrumentation

Instrumentation and conditions included

• Dionex ICS-2000 Reagent-Free IC System with EluGen EGC-KOH cartridge for on-line KOH eluent generation
• CR-ATC continuously regenerated anion trap for carbonate removal
• IonPac AG18 guard and AS18 analytical column (4 × 50 mm and 4 × 250 mm), 7.5 μm macroporous resin with alkanol quaternary ammonium latex functionalization
• Suppressor AutoSuppression ASRS ULTRA operated in recycle mode at 100 mA
• Eluent gradient from 22 to 40 mM KOH over 7–8 min, flow rate 1.0 mL/min, column temperature 30 °C, injection volume 25 μL, total run time 20 min

Sample pretreatment involved 0.45 μm filtration (0.20 μm for sewage) with optional C18 cartridge cleanup for domestic wastewater matrices.

Main Results and Discussion

Compared with the conventional AS4A column and carbonate eluent, the AS18 with KOH eluent showed:

• Improved resolution of fluoride from the void volume and clear separation of chloride, nitrite, bromide, sulfate, nitrate and phosphate peaks
• Enhanced capacity (285 μeq vs 20 μeq) allowing direct analysis of high ionic strength samples without dilution
• Excellent calibration linearity for all anions over 0.1–200 mg/L (r2 >0.999)
• Method detection limits between 1.6 and 5.7 μg/L calculated from seven replicates (MDL = σ·tS,99)
• Retention time RSD <0.15% and peak area RSD <0.8% from quality control standard injections
• Recoveries of 88–120% across drinking, surface, raw, industrial and domestic wastewater matrices, meeting EPA criteria

Benefits and Practical Applications

Implementation of the AS18 column with reagent-free eluent generation offers

• Automated high-purity hydroxide eluent production eliminating manual reagent preparation and carbonate contamination
• Robust suppression and stable baselines with the CR-ATC trap and ASRS suppressor
• Expanded calibration range reducing or eliminating sample dilutions for high sulfate or chloride waters
• Improved sensitivity and MDLs supporting compliance monitoring in drinking water, groundwater, wastewater and industrial discharges

Future Trends and Opportunities

Further developments may include coupling hydroxide‐selective columns with mass spectrometry detection for speciation studies, advanced trap column chemistries for ultralow background, and expanded applications in quality control laboratories and on-site monitoring platforms.

Conclusion

The combination of a high-capacity IonPac AS18 column and reagent-free, electrolytic potassium hydroxide eluent generation significantly enhances separation performance, sensitivity, and automation for the determination of inorganic anions in environmental waters under U.S. EPA Method 300.0. This approach streamlines compliance monitoring, reduces manual preparation steps, and improves data reliability.

Reference

• U.S. Environmental Protection Agency Method 300.0 Determination of Inorganic Anions in Water by Ion Chromatography 1993
• Greenberg A E Clesceri L S Eaton A D Standard Methods for the Examination of Water and Wastewater 18th Edition APHA 1992
• ASTM D4327-97 Standard Test Methods for Anions in Water by Chemically Suppressed Ion Chromatography ASTM International 1999
• Dionex Corporation Application Notes 133 and 135 Sunnyvale CA